chore: Update model references in README

Changed all instances of "gemini-2.0-flash" to "gemini-2.5-flash" in the example code snippets within the README.

PiperOrigin-RevId: 805949871

Author: hangfei

README.md

@@ -94,7 +94,7 @@ from google.adk.tools import google_search
 
 root_agent = Agent(
     name="search_assistant",
-    model="gemini-2.0-flash", # Or your preferred Gemini model
+    model="gemini-2.5-flash", # Or your preferred Gemini model
     instruction="You are a helpful assistant. Answer user questions using Google Search when needed.",
     description="An assistant that can search the web.",
     tools=[google_search]
@@ -109,13 +109,13 @@ Define a multi-agent system with coordinator agent, greeter agent, and task exec
 from google.adk.agents import LlmAgent, BaseAgent
 
 # Define individual agents
-greeter = LlmAgent(name="greeter", model="gemini-2.0-flash", ...)
-task_executor = LlmAgent(name="task_executor", model="gemini-2.0-flash", ...)
+greeter = LlmAgent(name="greeter", model="gemini-2.5-flash", ...)
+task_executor = LlmAgent(name="task_executor", model="gemini-2.5-flash", ...)
 
 # Create parent agent and assign children via sub_agents
 coordinator = LlmAgent(
     name="Coordinator",
-    model="gemini-2.0-flash",
+    model="gemini-2.5-flash",
     description="I coordinate greetings and tasks.",
     sub_agents=[ # Assign sub_agents here
         greeter,